A photographic material is generally produced by coating one or more photographic layer (light-sensitive layer) on a plastic film support. As the plastic film support, celluloses such as triacetyl cellulose (hereinafter referred to as "TAC") or polyesters such as polyethylene terephthalate (hereinafter referred to as "PET").
A photographic material is generally classified into two groups: a film in the form of a sheet such as an X-ray film, a film for photomechanical process or a cut film; and a film in the form of a roll such as a color or black-and-white film having a width of 35 mm or less which is incorporated into a cylindrical cartridge (patrone). The latter film is usually employed by charging in a camera for shooting. Recently, the film has been employed by incorporating in a camera as a disposable camera.
TAC is mainly employed as a material of a support for a roll film TAC has the features of high transparency and no optical anisotropy. Furthermore, the curl in a TAC film introduced during development is easily smoothed.
In contrast, when a photographic material having a support which does not have as good a curl smoothening property as TAC is used in the form of roll, scratching and poor focus occur during the printing step of forming an image on a photographic printing paper, or jamming occurs during feeding of the film in the printing step.
Of the photographic materials other than TAC, PET has excellent productivity, mechanical strength such as bending strength and dimension stability. Thus, PET has heretofore been considered to be substitutable for TAC. However, when PET is employed as a support of a photographic material, the resulting support film exhibits a strong curling tendency and the strong curl remains even after development. It is difficult to pull out a curved film by its edge from a patrone, and handling of the curved film after development is also difficult. Thus, the use of the PET in the field of photographic materials has been restricted, despite the above-mentioned excellent properties.
Photographic materials have been developed for a variety of applications. For instance, increase in feeding speed of a photographic film in a camera per each shooting, increase in image magnification and reduction in size of the camera have been desired. The desired characteristics can be obtained by use of a support having high bending strength, good dimensional stability and reduced thickness.
In a photographic film-incorporated camera (disposable camera), the above characteristics, particularly reduction in size of the camera, is desired. To satisfy this need, a diameter of a spool or vacant core of a wound up film in a supply room as well as a size of the cylindrical cartridge (patrone) must be reduced.
There are two problems to be solved in order to attain the size-reduction of the patrone:
One of the problems resides in the reduction of the modulus of elasticity in bending (concerned with brittleness) with reducing the thickness of the film.
The second problem resides in a strong curl tendency developed with elapse of time due to the reduction of the size of the spool (of the cartridge). In the conventional 135 system device, a roll film for 36 frames which is charged in the cartridge has a diameter of 14 mm as the smallest one. If the diameter is desired to be 12 mm or less, or further 9 mm or less, a high curl tendency is produced in the roll film to bring about various troubles in handling of the exposed film (roll film). For instance, if the exposed film taken out from a small-sized cartridge is developed in a mini-laboratory automatic developing machine, the film is curled up during handling in the machine to cause "uneven development". In more detail, the film is fixed to a leader at only one edge but not fixed at the other edge so that the film is curled up because of its high curl tendency, and therefore the developing solution is not sufficiently fed to the curled area of the film to cause the uneven development. Further, the curled portion of the film is crushed by a roller in the machine to be broken.
For the above reason, the photographic film-incorporated camera is available only as a 12-exposure film or a 24-exposure film. The 24-exposure film is usually wound around a vacant core of a diameter of 14 mm in a supply room. A photographic film-incorporated camera having a 36-exposure film is not available although it is desired.
As described above, in order to obtain such a photographic film-incorporated camera, the reduction of the bending strength with reducing the thickness of the film and the strong curl tendency which renders operation of pulling out the film edge (film-tongue-end pulling out operation) difficult are required to be improved.
U.S. Pat. No. 4,141,735 discloses a process for reducing core-set curling tendency and core-set curl of polymeric film elements (polyester such as PET) which comprises heating a sheet or roll of self-supporting, core-set-prone thermoplastic polymeric film, at a temperature in the temperature range of from about 30.degree. C. up to about. The glass transition temperature (Tg) of said polymer for 0.1 to 1500 hours. To reduce core-set curling tendency, the patent mainly uses PET film having excellent productivity, mechanical strength and dimension stability as described above.
In more detail, the PET film is extruded at 200.degree. C. or higher to be subjected to a heat treatment. In such heat treatment, the heated film usually is first stretched lengthwise 2 to 3 times its original length, and then similarly stretched widthwise. The stretching, which is known also as "cold drawing", is carried out at temperatures below the temperature of melting but above the glass transition temperature of the polymer. The resulting film is referred to as being biaxially-oriented. The cold drawing affects some change in the crystallinity of the polymer. Subsequently, to enhance the crystallinity and to increase the dimensional stability of the film, the biaxially-oriented polymeric film is "heated-set" by heating it near its crystallization point, while maintaining it under constant tension. The heating and tensioning also ensure that the heat-set film remains transparent upon cooling. The directionally oriented and heat-set film is then conventionally subjected to a subsequent heat treatment known as a "heat-relax" treatment. The heat-relax treatment is performed at approx. the creep temperature of the polymer (PET: 130.degree. C.). The use of the heat-relax treatment in such a overall continuous processes results in a still further improvement in the dimensional stability and surface smoothness of the finished film.
In the U.S. Pat. No. 4,141,735, the finished film is further subjected to the above-described heat treatment to reduce core-set curling tendency.
However, even if the film subjected to the heat treatment in the patent is employed for the photographic film-incorporated camera, the resultant camera is not sufficiently improved in the curling tendency. In more detail, a roll film for 36 frames used in the conventional 135 system device has a diameter (of spool) of 14 mm as the smallest one, while a roll film used in the photographic film-incorporated camera is desired to have a diameter smaller than 14 mm, preferably not larger than 12 mm. Hence, the roll film used in the photographic film-incorporated camera is further required to enhance bending strength and brittleness, i.e., property that there is no occurrence of both bending after keeping bending state and breaking by bending.
Further, the photographic film-incorporated camera requires handling of pulling out the film edge (film-tongue-end) because the edge is perfectly incorporated in the cartridge (patrone), which differs from a conventional camera. Therefore, the photographic film-incorporated camera needs to be further improved in the curling tendency compared with one of the conventional camera.